The rotational bearings used in the motor for driving the hard disc, compact disc (CD) and digital video disc (DVD) are ball bearings and fluid dynamic bearings.
The ball bearings have the shortcoming that the load to the bearing becomes greater when used for a long time, which may readily cause vibration and noise. In the fluid dynamic bearings, on the other hand, rotation of the shaft makes the flow of lubricating oil, which generates a pressure to support the rotation of the shaft. Therefore, the shaft and the bearing portion do not come in direct contact with each other, so that the frictional resistance can be reduced and the vibration and the noise are favorably low. Owing to those advantages, the fluid dynamic bearings have been frequently used in recent years.
Recently, the fluid dynamic bearings have been required to be smaller in size, have higher precision, and rotate at higher speeds. This necessarily requires the lubricating oil for the fluid dynamic bearings to have low viscosity, excellent heat resistance, sufficient stability against oxidation, low evaporation properties.
When the lubricating oil for fluid dynamic bearings is heated to high temperatures, for example by continuous rotation of the motor, the lubricating oil thermally expands to reduce the viscosity thereof. In this case, the bearing stiffness may unfavorably tend to deteriorate, which may cause the problem that the bearing becomes too unstable to support the load of the rotator. In consideration of this, the viscosity of the lubricating oil used in the fluid dynamic bearings is required to exceed a certain level within the high temperature region.
When the lubricating oil stands in a low temperature region, for example at the initiation of the motor, the viscosity resistance tends to increase during the rotation if the viscosity of the lubricating oil is high. This will disadvantageously result in the increase of electric power loss in the motor. In consideration of this, it is required to minimize the increase of viscosity of the lubricating oil even when the lubricating oil is left at low temperatures. Namely, the viscosity of the lubricating oil used in the fluid dynamic bearing is required to be less changed when the temperature varies.
In addition, the rotating device generates static electricity, which is accumulated on the side facing to the device. To prevent the charged surface from discharging (overcurrent), the lubricating oil is also required to have antistatic properties.
JP 4160772 discloses di-n-octylate of 2,4-diethyl-1,5-pentanediol as the lubricating oil for the fluid dynamic bearing. This ester is reported to show more satisfactory results in terms of the viscosity index, low-temperature fluidity, thermal stability and low evaporation properties in a wider temperature range, which last a longer period of time when compared with poly α-olefins obtainable from polymers of 1-decene through hydrogenation, 2-ethylhexyl esters of adipic acid and sebacic acid, neopentyl glycol, pentaerythritol and the like. However, di-n-octylate of 2,4-diethyl-1,5-pentanediol is still insufficient in the thermal stability and the low evaporation properties at high temperatures.